The cost of a machining operation which requires the use of cutting tools is highly dependent on the wear rate of the tool. Consequently, attempts have been made in the past to measure tool wear directly, and to assess the useful length of life of a tool on the basis of tool wear, primarily on the basis of chamfer wear on the relief or clearance side of the tool. An experienced machine operator will be able to judge roughly with the naked eye whether or not the tool needs to be changed.
In present times automatization has progressed to such lengths that there is often no operator available, and consequently attempts have been made, for instance, to work with automated wear measuring methods with the use of image processing systems, which is an expensive and slow solution to the problem. It is necessary with such machines to halt machining work while the measurements are being taken. Attempts have also been made to measure tool wear indirectly, by measuring prevailing cutting forces, the changes in which forces are contingent on wear on the tool. The relationships, however, are not always specific and consequently the method is not reliable.
A third method involves measuring cutting temperatures, e.g. with the aid of a thermoelement inserted into a bore drilled in the tool. This results in mechanical weakening of the tool, however, and it is very difficult to drill holes in the extremely hard material from which such tools are made and to position the holes with precision. Tool temperatures are measured in on attempt to calculate the temperature of the cutting tool in the critical zone around the cutting edge--the contact surfaces between tool and chip or swarf and tools and workpiece respectively.
For mechanical strength reasons, it is necessary to place the thermoelement at a relatively long distance from the actual cutting edge and the temperatures measured will differ radically from the temperature which actually prevails at the contact surface and which determines the wear rate of the tool.